In modem electronics, it is common for an integrated circuit to generate a variety of signal types. An integrated circuit may, for example, generate data signals that convey information at a high signal rate, and may also generate arbitration signals that have a substantially lower signal rate and are useful for controlling other devices. Where signals having substantially different signal rates utilize the same signal path, various problems may arise due to the difference in signal rates. For example, a false or spurious signal commonly known as a glitch may occur in an output signal on the signal path due to the difference in signal rates.
One particularly important example occurs in computer systems having a bus that interconnects various components in accordance with IEEE Standard 1394 of the Institute of Electrical and Electronics Engineers. In this context, signals that have a high signal rate may include a data signal that conveys data in a signal rate range between about 100 MHz to 300 MHz, and may also include a strobe signal at substantially the same signal rate as the data signal that a receiving device may use to maintain synchronization with the data signal. The signals that have a low signal rate may include an arbitration signal having a signal rate of about 50 MHz. Such arbitration signals are specified in IEEE Standard 1394 for controlling operation of the bus and various devices connected to the bus.
One conventional approach for switching signals that have different signal rates onto the same signal path is to connect the signal sources of the signals to the signal path by means of a conventional multiplexor. If the maximum difference in signal rates between the signal sources exceeds about 100 MHz, this conventional approach does not operate properly. The difference in signal rates typically causes the multiplexor to generate a glitch at output. The glitch may be interpreted as being part of a data signal, and consequently may generate data errors. A glitch occurring when the multiplexor switches an arbitration signal onto the bus may be interpreted as being part of the arbitration signal, and could cause a receiving device to malfunction. Glitches occur at or near the beginning of a switched signal, and thus may interfere with signal prefixes, such as various signal prefixes required by IEEE Standard 1394 to indicate transition between signal types.
A further problem in this field is a one-cycle delay introduced to data and strobe signals by a conventional non-memoryless encoding circuit and method described in IEEE Standard 1394 for generating strobe signals. More particularly, IEEE Standard 1394 describes a conventional encoder for encoding data signals by means of recursion to generate strobe signals. This conventional encoder supplies the data signal to one input of a first XOR gate, and to the D input of a first D flip-flop. A delayed data signal supplied by the Q output of the first D flip-flop is fed back to the other input of the first XOR gate, and the output of the first XOR gate drives the first input of a second XOR gate. The output of the second XOR gate drives the D input of a second D flip-flop. The Q output of the second D flip-flop supplies the strobe signal. The /Q output supplies an inverted strobe signal that is fed back to the second input of the second XOR gate. Both D flip-flops are clocked by the same clock signal.
Clearly, this encoding is recursive due to operation of the D flip-flops. The encoding requires feedback from two sources. Moreover, the data signal must be delayed by at least one clock cycle to generate the strobe signal, and the data signal supplied to the bus will actually be a delayed data signal that is essentially a copy of the original data signal delayed by at least one full clock cycle. This added latency slows down the transmission of data to other devices that are connected to the bus, and thus is highly undesirable. There is thus a continuing need in the electronics arts for an improved circuit and an improved method for connecting a signal path to signal sources that supply signals at substantially different signal rates. Such a circuit and method preferably prevents output glitches from arising during switching operations. It is highly desirable for such circuit and method to be implemented in conformance with IEEE Standard 1394, and to support data and strobe signal rates that may exceed 200 MHz. An improved switching circuit is needed to drive the output buffers on an integrated circuit (IC) chip. Such an improved circuit preferably requires minimal space on the IC chip to implement.
There is also a need for an improved encoder and encoding method for encoding digital signals in parallel. Preferably such encoder and encoding method can be implemented to generate strobe signals in accordance with IEEE Standard 1394. To minimize latency of data and strobe signals, it is highly desirably to generate such strobe signals without use of recursion. Further it is highly desirable for at least one embodiment of the encoder to supply strobe signals according to IEEE Standard 1394 without introducing latency penalties.